Considerable efforts have been expended to develop ceramic dielectric materials that are capable of being produced by sintering at sufficiently low temperatures that relatively inexpensive metals can be used as electrodes for fabricating capacitors. Electrode metals such as palladium, silver/palladium alloy, platinum and silver/platinum alloy have been used in order to withstand the high sintering temperatures required to produce many types of ceramic capacitors. These electrode materials, due to the presence of palladium or platinum, constitute a significant expense in the fabrication of a capacitor. Less expensive materials such as silver have lower melting temperatures (silver melts at 961.degree. C.) and thus, to enable fabrication of a viable ceramic capacitor, the dielectric material must be capable of being formed at sintering temperatures sufficiently low that the electrode material does not melt or unduly soften and does not migrate into the dielectric material and adversely affect the performance of the capacitor.
In order to make, especially on a reproducible basis as required for commercial production, ceramic capacitors, the sintering should produce a uniform, dense ceramic body. Characteristic of insufficiently sintered ceramics is the presence of open, or porous, structures and incomplete formation of perovskite phases. Modifications can be made to precursor compositions, e.g., by the inclusion of sintering aids and changes in perovskite phase-forming components, in attempts to provide a composition capable of being sintered at lower temperatures. However, these modifications must be carefully made to avoid untoward effects on the performance of the capacitor.
Demanding performance standards exist for capacitors. Minimum specifications frequently include a dielectric constant of at least about 10,000 at 25.degree. C., a dissipation factor of less than about 3 percent, and high insulation resistance (at least 10.sup.12 ohm-cm at 25.degree. C.). Moreover, extremely high reproducability, reliability and freedom from failures with operation are essential. See, for instance, Electric Industries Association Standard RS-198-C (November 1983) for Z5U-type capacitors.
Park, et al., in U.S. Pat. No. 4,550,088, disclose lead oxide-based dielectric ceramic materials which are capable of being sintered at temperatures in the range of 900.degree. to 1000.degree. C. and which contain 64.32 to 66.20 weight percent PbO, 2.49 to 6.67 weight percent Fe.sub.2 O.sub.3, 24 18 to 24.14 weight percent Nb.sub.2 O.sub.5, 2.09 to 3.59 weight percent MgO, 0.025 to 0.10 weight percent MnO.sub.2 and 0.20 to 1.50 weight percent of a sintering aid additive having a melting point of no more than 775.degree. C. and a resistivity at 25.degree. C. of at least 10.sup.13 ohm-cm, wherein the sintering aid contains one or more of GeO.sub.2, SiO.sub.2, Bi.sub.2 O.sub.3, CdO, ZnO, Al.sub.2 O.sub.3, CuO and B.sub.2 O.sub.3 chemically combined with at least 20 weight percent of PbO.
Furukawa, et al., in U.S. Pat. No. 4,216,102 disclose a ceramic composition containing a solid solution of Pb(Mg.sub.1/3 NB.sub.2/3)O.sub.3 -Pb(Fe.sub.1/2 NB.sub.1/2)O.sub.3 which is said to be capable of being formed by sintering at 800.degree. to less than 1000.degree. C. The patentees disclose various additives such as Pb(Mn.sub.2/3 W.sub.1/3).sub.O.sub.3, Pb (Mn.sub.1/2 W.sub.1/2)O.sub.3, Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3, Li.sub.2 O, Cr.sub.2 O.sub.3 and CeO.sub.2. The basic ceramic composition consists of 67.99 to 68.58 weight percent PbO, 0.61 to 8.51 weight percent Fe.sub.2 O.sub.3, 1.23 to 3.92 weight percent MgO and 22.27 to 26.89 weight percent Nb.sub.2 O.sub.5.
Yonezawa, et al., in U.S. Pat. No. 4,078,938, disclose ceramic compositions which purportedly can be sintered at temperatures below about 1000.degree. C. which contain Pb(Fe.sub.2/3 W.sub.1/3).sub.x (Fe.sub.1/2 Nb.sub.1/2).sub.1-x O.sub.3 where 0.2 to 0.5.
Fujiwara, et al., in U.S. Pat. No. 4,287,075 disclose a ceramic composition having a solid solution structure of Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 -Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3. This composition comprises 64.80 to 68.58 weight percent PbO, 0 to 11 46 weight percent Fe.sub.2 O.sub.3, 0.3 to 3.92 weight percent MgO, 9.65 to 26.89 weight percent Nb.sub.2 O.sub.5 and 0 to 16.83 weight percent WO.sub.3. Additives include MnO, Pb(Mn.sub.2/3 W.sub.1/3)O.sub.3, Pb(Mn.sub.1/2 W.sub.1/2)O.sub.3, Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3, Pb(Mn.sub.1/3 Ta.sub.2/3)O.sub.3, Li.sub.2 O, Cr.sub.2 O.sub.3 and CeO.sub.2.
Japanese Patent Application Kokai No. 57-208004 describes ceramic dielectric material especially useful for laminated capacitors containing Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -CdTiO.sub.3 -Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3. Exemplified additives are MnO.sub.2, CeO.sub.2, Cr.sub.2 O.sub.3 and CoO. Japanese Patent Application Kokai No. 57-189407 describes also ceramic compositions useful for dielectrics for laminated capacitors containing Pb(Mg.sub.1/3 Nb.sub.2/3) O.sub.3 -CdTiO.sub.3. Additives specifically described are MnO.sub.2, Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, CeO.sub.2 and WO.sub.3. The applicants state that PbO.MgO are to be used in excess. When the components are not within the specific ranges described in the publication, the materials are said not to insert or have low dielectric constants or have large dielectric loss.
Takagi, et al., in U.S. Pat. No. 4,812,426 disclose lead powders containing perovskite-type lead-containing oxides The powder is prepared by (a) mixing and calcining a material selected from a broad group of components, (b) adding a lead substance, and (c) calcining the mixture. The materials in step (a) include oxides containing titanium, zirconium, lithium, copper, magnesium, nickel, barium, calcium, strontium, zinc, manganese, cobalt, tin, iron, cadmium, antimony, aluminum, rare earth metals, indium, selenium, niobium, tantalum, bismuth, tungsten, tellurium, rhenium and mixtures thereof.
Thomas in U.S. Pat. No. 4,582,814, discloses dielectric compositions having 95.5 to 99.4 weight percent of (Sr.sub.x Pb.sub.l-x TiO.sub.3).sub.a (PbMg.sub.r W.sub.s O.sub.3).sub.b and 4.5 to 6.0 weight percent of a mixture of metal oxides or precursors thereof consisting essentially of (1) binary oxide of a transition metal selected from the group consisting of Co, Ni, Cr, Mn and mixtures thereof, (2) Cd titanate, Zn titanate or mixtures thereof, and (3) a polynary oxide selected from the group consisting of Cd zirconate, Zn zirconate, Cd stannate, Zn stannate and mixtures thereof.
Maher in U.S. Pat. No. 4,027,209, discloses dielectric material comprising Pb.sub.l-x La.sub.x (Zr.sub.y Ti.sub.l-y).sub.l-x/4 O.sub.3 which is doped with 0.1 to 10 weight percent silver. The capacitor may contain low temperature glass which may comprise glass formers selected from B.sub.2 O.sub.3, SiO.sub.2, GeO.sub.2, Al.sub.2 O.sub.3, ZnO, CdO, Bi.sub.2 O.sub.3 and PbO. United Kingdom published patent application 2,039,877A discloses a similar composition but further containing barium titanate. See also, U.S. Pat. No. 4,266,265 in which high cadmium-content silicates are used as flux in similar capacitor formulations.
Lewis in U.S. Pat. No. 2,864,713 discloses various niobate compounds having dielectric properties. The compositions have the formula (l-x)L.sub.2 O-xMO-R.sub.2 O.sub.5 wherein L is sodium which may be partially replaced by potassium, M is cadmium or lead and R is niobium which may be partially replaced by tantalum.
Walker, et al., in U.S. Pat. No. 2,277,734, disclose as dielectric materials, the product of firing at high temperature a composition of 80 to 97 percent titanium dioxide and the balance of preformed titanate of calcium, strontium, barium, cadmium, zinc and divalent lead.